1. Technical Field
The present invention relates to a rotary type electric shaver in which an inner cutter(s) rotates while making a sliding contact with an interior surface of an outer cutter(s), so that whiskers (or hair) that advance into hair introduction openings extending in substantially a radial direction in the top portion of the outer cutter is cut by the inner cutter(s).
2. Description of the Related Art
In electric rotary shaves, when, during shaving, plural slit-shaped hair introduction openings (or “slits”) of the outer cutter of a shaver is firmly pressed against skin, the skin would enter into the slits and be cut by the inner cutter (occasionally cut too deeply), causing a burning feeling in the skin after finishing the shaving. This sort of problem can be prevented by using outer cutters which are made thick. Here, making the outer cutter thick means to increase the dimension of the thickness of the ribs that are on both sides of each radial direction slit (the dimension in the direction parallel to the rotational axis of the inner cutter). However, increase of the thickness of the outer cutter results in expansion of the distance between the skin and the inner cutter, causing another problem that it is not possible to cut hair deep enough.
A proposal for preventing the problems described above is disclosed in, for instance, Japanese Patent Application Laid-Open (Kokai) No. H11-4980. In the shaver disclosed in this related art, as shown in FIG. 7, a plurality of concentric circular inner cutter running tracks 2a and 2b are formed in the outer cutter 1; and slit-shaped blades (ribs) 3a are formed in the outer inner cutter running track 2a, and slit-shaped blades (sits) 3b are formed in the inside inner cutter running track 2b, with the outside slit-shaped blades (ribs) 3a are thicker than the inside slit-shaped blades (ribs) 3b. 
Generally, when the outer cutter(s) (or the shaver) is moved around while being pressed against the shaving part of the face during the shaving, the skin tends to come into contact with the outer periphery of the outer cutter(s), and wrinkles occur; and the pressing force becomes larger at the outer peripheral side of the outer cutter.
In view of this problem, in the electric shaver disclosed in Japanese Patent Application Laid-Open (Kokai) No. H11-4980, the peripheral side of the outer cutter is made thicker to prevent excessive deep shaving. Since the pressing force becomes smaller in the inner peripheral side, the inner peripheral side of the outer cutter is made thinner, so that shaving with appropriate depth can be made on the inner peripheral side.
Japanese Patent Application Laid-Open (Kokai) No. H7-185148 teaches an electric shaver in which the sliding surface where the cutter blade of the inner cutter make a sliding contact with the inner cutter running track of the outer cutter has a constant thickness in the vertical cross-section (that includes the rotational center axis) and projects upward in a convex arc shape.
The electric shaver disclosed in Japanese Patent Application Laid-Open (Kokai) No. H11-4980 is designed based upon a use state of a shaver in which, when the outer cutter (or the shaver) is moved around while being firmly pressed against the skin, the skin wrinkles at the outer peripheral edge of the outer cutter, and the pressing force becomes larger. In an actual use, however, the outer cutter (or the shaver) is often moved around while touching the skin lightly; and in this case, large wrinkles would not occur on the outer peripheral edge of the outer cutter, and the shaving performance tends to be poor.
On the other hand, in the electric shaver disclosed in Japanese Patent Application Laid-Open (Kokai) No. H7-185148, the sliding surface where the cutter blades of the inner cutter make a sliding contact with the inner cutter running track of the outer cutter has a curvature that is convex upward (in an imaginary radial cross-section that includes the rotational center axis of the inner cutter). Accordingly, the problem is that manufacturing of the outer cutter is not easy, and the productivity is poor. Furthermore, the whetstone that grinds the sliding surface (cutting surface) that has an outwardly convex shape must be processed in advance so that the whetstone has a grinding surface that matches the curvature of the curved sliding surface; as a result, the durability of the whetstone also becomes an issue.